1. Field of the Invention
The present invention relates to an optical system for optical pickup, which is used in an optical pickup device which performs at least one of reading, writing, and erasing of information on an information storage medium such as a plurality of optical discs having different light-transmitting layer thicknesses.
2. Description of the Background Art
Due to practical application of a blue-violet semiconductor laser, Blue-ray Disc (registered trademark; hereinafter, referred to as “BD”), which is a high-density and high-capacity optical information storage medium (hereinafter, also referred to as “optical disc”) having the same size as those of CD (Compact Disc) and DVD, has been put into practical use.
The thickness of the light-transmitting layer of CD is 1.2 mm, the wavelength of the laser beam for performing recording or reproducing is about 785 nm, the numerical aperture NA of the objective lens is 0.45 to 0.52, and the recording capacity is about 650 MByte.
The thickness of the light-transmitting layer of DVD is about 0.6 mm, the wavelength of the laser beam for performing recording or reproducing is about 660 nm, the numerical aperture NA of the objective lens is 0.60 to 0.66, and the recording capacity of an information recording surface of one layer is about 4.7 GByte. A single-layer disc having a single information recording surface and a two-layer disc having two information recording surfaces have been put into practical use.
The thickness of the light-transmitting layer of BD is about 0.1 mm, the wavelength of the laser beam for performing recording or reproducing is about 405 nm, the numerical aperture NA is about 0.85, and the recording capacity of an information recording surface of one layer is about 25 GByte. As BD as well, a single-layer disc having a single information recording surface and a two-layer disc having two information recording surfaces have been put into practical use.
As in BD, when recording or reproducing of information is performed on a plurality of information recording surfaces, the thickness of the light-transmitting layer is different at each information recording surface. On an information recording surface which is located at a depth different from the optimum light-transmitting layer thickness for the objective lens (the thickness of the light-transmitting layer at which a third-order spherical aberration is the minimum when parallel light is incident on the objective lens), a third-order spherical aberration corresponding to the difference from the optimum light-transmitting layer thickness occurs. In BD, with respect to a light-transmitting layer thickness error of 10 μm, a third-order spherical aberration of about 100 mλ occurs. Thus, in general, an optical pickup device which performs recording or reproducing on BD includes a means for compensating a third-order spherical aberration.
For example, Japanese Laid-Open Patent Publication No. 11-259906 discloses an optical pickup device which includes a collimating lens mounted on a collimating lens actuator and located between a light source and an objective lens and which moves the collimating lens along the optical axis direction so as to cancel a third-order spherical aberration caused by a thickness difference of the light-transmitting layer, thereby changing the divergence angle or convergence angle of a laser beam incident on the objective lens.
Meanwhile, many optical pickup devices for high-density optical disc such as BD, which use a short-wavelength laser beam and a high-NA objective lens, include a means for compensating a third-order coma aberration which occurs due to tilt of the optical disc (hereinafter, also referred to as “disc tilt”). For example, a method in which an objective lens mounted on an objective lens actuator is tilted in the radial direction of the optical disc, and a method using a liquid crystal element have been put into practical use.
In recent years, various optical pickup devices have been proposed in which a plurality of objective lenses is mounted to ensure compatibility with CD, DVD, and BD.
FIG. 20 is a diagram showing an example of an optical pickup device (optical head) configured with two objective lenses.
The optical pickup device 140 shown in FIG. 20 includes a blue-violet laser beam source 101 which emits a blue-violet laser beam, a relay lens 102, a polarizing beam splitter 103, a collimating lens 104, a plate-shaped mirror 105, a quarter wavelength plate 106, a diffractive lens 107, an objective lens 108, an objective lens actuator 109, a two-wavelength light source 111 which selectively emits a red laser beam and an infrared laser beam, a diffraction grating 112, a plate type beam splitter 113, a collimating lens actuator 114, a wedge-shaped mirror 115, a quarter wavelength plate 116, a compatible objective lens 118, a detection hologram 121, a detection lens 122, a photo detector 123, and a front monitor sensor 124. The objective lens 108 is dedicated for a BD 90 having information recording surfaces L0 and L1 of two layers, and the compatible objective lens 118 is shared by a DVD 70 and a CD 80.
An operation of the optical pickup device 140 which is performed when recording or reproducing is performed on the BD 90 will be described. A blue-violet laser beam with a wavelength of about 405 nm, which is emitted from the blue-violet laser beam source 101, is converted by the relay lens 102 into diverging light and is incident as S-polarized light on the polarizing beam splitter 103. The laser beam reflected by the polarizing beam splitter 103 is converted by the collimating lens 104 into substantially parallel light, passes through the wedge-shaped mirror 115, and is reflected by the plate-shaped mirror 105 to be bent toward the quarter wavelength plate 106. A portion of the laser beam incident on the plate-shaped mirror 105 passes through the plate-shaped mirror 105 and is incident on the front monitor sensor 124. On the basis of output of the front monitor sensor 124, output of the blue-violet laser beam source 101 is controlled. The laser beam reflected by the plate-shaped mirror 105 is converted by the quarter wavelength plate 106 into circularly polarized light, then passes through the diffractive lens 107, and is converged by the objective lens 108 as a light spot on either one of the information recording surface L0 or L1 of the BD 90.
The blue-violet laser beam reflected by the information recording surface L0 or L1 of the BD 90 passes through the objective lens 108 and the diffractive lens 107 again, is converted by the quarter wavelength plate 106 into linearly polarized light having a polarization plane different from that in the path to the BD 90, and then is reflected by the plate-shaped mirror 105. The light reflected by the plate-shaped mirror 105 passes through the wedge-shaped mirror 115 and the collimating lens 104, then is incident as P-polarized light on the polarizing beam splitter 103, passes through the polarizing beam splitter 103, and is guided to the photo detector 123 via the plate type beam splitter 113, the detection hologram 121, and the detection lens 122. The laser beam detected by the photo detector 123 is photoelectrically converted and then subjected to predetermined arithmetic processing to generate a focus error signal for following surface run-out of the BD 90 and a tracking error signal for following decentering of the BD 90.
Next, an operation of the optical pickup device 140 which is performed when recording or reproducing is performed on the DVD 70 will be described. A red laser beam with a wavelength of about 660 nm, which is emitted from the two-wavelength light source 111, is split by the diffraction grating 112 into a main beam, which is zeroth order light, and sub-beams which are ±1st order diffracted light. The main beam and the sub-beams are incident as S-polarized light on the plate type beam splitter 113, are reflected by the plate type beam splitter 113, pass through the polarizing beam splitter 103, and are converted by the collimating lens 104 into substantially parallel light. The light emitted from the collimating lens 104 is reflected by the wedge-shaped mirror 115 to be bent toward the quarter wavelength plate 116. A portion of the laser beam incident on the wedge-shaped mirror 115 passes through the wedge-shaped mirror 115 and the plate-shaped mirror 105 and is incident on the front monitor sensor 124, and output of the red laser beam of the two-wavelength light source 111 is controlled on the basis of output of the front monitor sensor 124. The laser beam reflected by the wedge-shaped mirror 115 is converted by the quarter wavelength plate 116 into circularly polarized light and then converged by the objective lens 118 as a light spot on an information recording surface of the DVD 70.
The red laser beam reflected by the information recording surface of the DVD 70 passes through the objective lens 118 again, is converted by the quarter wavelength plate 116 into linearly polarized light having a polarization plane different from that in the path to the DVD 70, and then is reflected by the wedge-shaped mirror 115. The light reflected by the wedge-shaped mirror 115 passes through the collimating lens 104, then is incident as P-polarized light on the polarizing beam splitter 103 and the plate type beam splitter 113, passes therethrough, and is guided to the photo detector 123 via the detection hologram 121 and the detection lens 122. The laser beam detected by the photo detector 123 is photoelectrically converted and then subjected to predetermined arithmetic processing to generate a focus error signal for following surface run-out of the DVD 70 and a tracking error signal for following decentering of the DVD 70.
Next, an operation of the optical pickup device 140 which is performed when recording or reproducing is performed on the CD 80 will be described. An infrared laser beam with a wavelength of about 785 nm, which is emitted from the two-wavelength light source 111, is split by the diffraction grating 112 into a main beam, which is zeroth order light, and sub-beams which are ±1st order diffracted light. The main beam and the sub-beams are reflected by the plate type beam splitter 113, pass through the polarizing beam splitter 103, are converted by the collimating lens 104 into substantially parallel light, and are reflected by the wedge-shaped mirror 115 to be bent toward the quarter wavelength plate 116. A portion of the laser beam incident on the wedge-shaped mirror 115 passes through the wedge-shaped mirror 115 and the plate-shaped mirror 105 and is incident on the front monitor sensor 124, and output of the infrared laser beam of the two-wavelength light source 111 is controlled on the basis of output of the front monitor sensor 124. The laser beam reflected by the wedge-shaped mirror 115 passes through the quarter wavelength plate 116 and is converged by the objective lens 118 as a light spot on an information recording surface of the CD 80.
The infrared laser beam reflected by the information recording surface of the CD 80 passes through the objective lens 118 and the quarter wavelength plate 116 again, is reflected by the wedge-shaped mirror 115, and passes through the collimating lens 104. The light emitted from the collimating lens 104 passes through the polarizing beam splitter 103 and the plate type beam splitter 113 and is guided to the photo detector 123 via the detection hologram 121 and the detection lens 122. The laser beam detected by the photo detector 123 is photoelectrically converted and then subjected to predetermined arithmetic processing to generate a focus error signal for following surface run-out of the CD 80 and a tracking error signal for following decentering of the CD 80.
In order to further increase the capacity of an optical disc, it is considered to provide information recording surfaces of multiple layers, which are three layers or more, in a high-density optical disc such as BD. In an optical disc having a plurality of information recording surfaces, predetermined intervals between the information recording surfaces have to be ensured in order to reduce influence of reflected light (stray light) from an adjacent information recording surface (crosstalk of an information signal, offset of a servo signal, and the like). Therefore, in a multilayer optical disc having information recording surfaces of three layers or more, the interval between the information recording surface at which the thickness of the light-transmitting layer is the largest and the information recording surface at which the thickness of the light-transmitting layer is the smallest has to be made larger than that in a conventional two-layer disc.
When recording or reproducing is performed on such a multilayer optical disc, a third-order spherical aberration which occurs in proportion to a difference from the optimum light-transmitting layer thickness for the objective lens increases. Thus, in an optical pickup device for multilayer optical disc, the range in which a collimating lens is moveable has to be made larger than that in the conventional art, to be able to compensate a larger third-order spherical aberration.
In the conventional optical pickup device 140 shown in FIG. 20, in performing recording or reproducing on the BD 90, when the collimating lens 104 is moved along the optical axis direction in order to compensate a third-order spherical aberration which occurs in accordance with the thickness of the light-transmitting layer, non-parallel light (diverging light or converging light) is incident on the wedge-shaped mirror 115 and a third-order astigmatism occurs.
FIG. 21 shows a result obtained by calculating, for each vertex angle α of the wedge-shaped mirror 115, how a third-order astigmatism changes when the collimating lens is moved in accordance with the thickness of the light-transmitting layer. In FIG. 21, the horizontal axis indicates the thickness of the light-transmitting layer, and the vertical axis indicates a third-order astigmatism amount. The calculation conditions are as follows.
Designed wavelength for an objective lens: 405 nm
Designed light-transmitting layer thickness for the objective lens: 87.5 μm
Focal length of the objective lens: 1.3 mm
Numerical aperture (NA) of the objective lens: 0.855
Thickness of a wedge-shaped mirror: 1.0 mm
Refractive index of the wedge-shaped mirror: 1.53
As shown in FIG. 21, the amount of third-order astigmatism which occurs when the collimating lens is moved in accordance with the thickness of the light-transmitting layer changes depending on the vertex angle α of the wedge-shaped mirror 115 through which a laser beam passes. It is recognized that when the vertex angle α of the wedge-shaped mirror 115 is zero, namely, its incident surface and its reflecting surface are parallel to each other, the change amount of third-order astigmatism is the smallest.
Meanwhile, it is known that when the thickness of the light-transmitting layer of the optical disc changes, an amount of third-order coma aberration which occurs at disc tilt and an amount of third-order coma aberration which occurs at objective lens tilt (hereinafter, also referred to as “lens tilt”) changes depending on the thickness of the light-transmitting layer of the optical disc. An amount of third-order coma aberration which occurs when the optical disc is tilted by a predetermined angle (at disc tilt) increases in proportion to the thickness of the light-transmitting layer, and an amount of third-order coma aberration which occurs when the objective lens is tilted by a predetermined angle (at lens tilt) decreases as the thickness of the light-transmitting layer increases.
Therefore, when light is converged on an information recording surface at which the thickness of the light-transmitting layer is large, the objective lens has to be greatly tilted in order to compensate a third-order coma aberration which occurs due to disc till. However, in general, when the objective lens is tilted, a third-order astigmatism occurs in response to the tilt of the objective lens.
In a general optical disc device, as shown in FIG. 22, an optical system is disposed such that the optical axis of the collimating lens 104 coincides with the tangential direction of an optical disc (the CD 70, the DVD 80, the BD 90, or the like). As shown in FIG. 22, a laser beam incident from the tangential direction of the optical disc is reflected by the wedge-shaped mirror 115 and converged by the objective lens 108, or is reflected by the plate-shaped mirror 105 and converged by the compatible objective lens 118. By providing such an arrangement, as shown in FIG. 23, it is made easy to access the innermost portion of the optical disc, and a portion of the optical head which protrudes when the optical head accesses the outermost portion of the optical disc is small.
However, when the optical system is disposed such that the optical axis of the collimating lens 104 coincides with the tangential direction of the optical disc, a third-order astigmatism (first astigmatism) which occurs when the collimating lens 104 is moved along the optical axis direction to compensate a third-order spherical aberration and a third-order astigmatism (second astigmatism) which occurs when the objective lens is tilted in the radial direction of the optical disc to compensate a third-order coma aberration include components of the same directions (0 deg/90 deg directions) and have the same polarity.
As described above, when recording or reproducing is performed on an information recording surface at which the thickness of the light-transmitting layer is large, both the first astigmatism and the second astigmatism increase. Therefore, particularly, in an optical pickup device for a multilayer optical disc having information recording surfaces of three layers or more, there is fear that addition of the first and second astigmatisms greatly influences recording or reproducing.